Circuit board with trace configuration for high-speed digital differential signaling

ABSTRACT

Trace configurations for carrying high-speed digital differential signals provide for reduced conduction loss and improved signal integrity. In one embodiment, a circuit board has a first set of conductive traces disposed on non-conductive material, and a second set of conductive traces parallel to the first set and disposed within the conductive material. The second set is separated from the first set by non-conductive material. Corresponding traces of the first and second sets may be in a stacked configuration. In other embodiments, conductive material may be provided between corresponding traces of the first and second sets resulting in an “I-shaped” or “U-shaped” cross-section. In yet other embodiments, the trace configurations have “T-shaped” and “L-shaped” cross-sections.

This application is a divisional of U.S. patent application Ser. No.10/167,904, filed on Jun. 12, 2002, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention pertains to high-speed digital differentialsignaling and in one embodiment, to circuit boards with traces forcommunicating high-speed digital differential signals.

BACKGROUND OF THE INVENTION

Digital signals, including digital differential signals, are used forsignal transmission on circuit boards, integrated circuit packages,interposer substrates and motherboards to help protect a signal frompicking up external noise. Digital differential signals are also used incomputer systems and communication systems such as a local area network(LAN) system. A digital differential signal has two components that are180 degrees out-of-phase with each other. The signal componentstransition between digital values of zero and one which may berepresented by particular voltages. Digital differential signals areconventionally transmitted using a pair of traces spaced closelytogether on a circuit board or substrate. Each trace of the pair carriesone of the components of the digital differential signal. FIG. 1illustrates a cross-section of a portion of a circuit board with a pairof traces for communicating digital differential signals in accordancewith the prior art. Circuit board 100 has trace pair 102 on insulatingsubstrate 104. Conductive ground plane 106 is on a side of substrate 104opposite to traces 102. Trace pair 102 illustrates a pair of closelyspaced traces that have been conventionally used to carry digitaldifferential signals. Because the signal components are out-of-phase,coupling between the traces reduces the signal's susceptibility toexternal noise. The darkened/solid areas (e.g., trace pairs 102 andground plane 106) illustrated in the informal drawings are conductivematerial while the non-darkened areas may be a non-conductive orinsulating material.

Semiconductor devices, computers, and other elements in digital systemscontinue to increase their operating data rate requiring thecommunication of digital differential signals of increasingly highertransition-rates. The transition-rate refers to the rate at which adigital signal transitions between states. In the near future, digitaldevices may require high-speed input/output (I/O) communications usingdigital differential signals, which may exceed rates of onegiga-transition per second, and even rates of even ten giga-transitionper second. One problem with conventional circuit board trace pairs isthat as the transition-rate increases, conduction loss also increases.Furthermore, as the transition-rate increases, signal integritydegrades. Conventional circuit board trace pairs for carrying digitaldifferential signals may have upper limits of less than one or tengiga-transitions per second and are therefore generally unsuitable forcarrying higher rate digital differential signals. Digital differentialsignals above one and especially above ten giga-transitions per secondwhen communicated over conventional trace pairs results in unacceptablyhigh conduction loss and reduction in signal quality and integrity.

One technique that has been used to help reduce the increase inconduction loss associated with high transition-rate digitaldifferential signals is increasing the trace width. Increased tracewidths, however, reduce signal routing ability and consume more area ona circuit board, substrate or package, for example. Increased tracewidth also results in higher dielectric loss due to increasedcapacitance, which contributes to the reduction in signal quality andintegrity.

Thus there is a need for improved communication of high-speed digitaldifferential signals. There is also a need for a circuit board havingtraces suitable for communication of high-speed digital differentialsignals. There is also a need for traces for communicating high-speeddigital differential signals with reduced conduction loss. There is alsoa need for traces for communicating high-speed digital differentialsignals with reduced dielectric loss. There is also a need for tracesfor communicating high-speed digital differential signals that helpmaintain signal integrity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.However, a more complete understanding of the present invention may bederived by referring to the detailed description when considered inconnection with the figures, wherein like reference numbers refer tosimilar items throughout the figures and:

FIG. 1 illustrates a cross-section of a portion of a circuit boardhaving trace pairs for communicating digital differential signals inaccordance with the prior art;

FIGS. 2, 3, 4, 5 and 6 illustrate cross-sections of a circuit boards forcommunicating high-speed digital differential signals in accordance withvarious embodiment of the present invention;

FIGS. 7A, 7B and 7C illustrate the fabrication of a circuit board inaccordance with an embodiment of the present invention; and

FIG. 8 is a highly simplified functional block diagram of a system inaccordance with an embodiment of the present invention.

The description set out herein illustrates the various embodiments ofthe invention and such description is not intended to be construed aslimiting in any manner.

DETAILED DESCRIPTION

The present invention relates to circuit boards having traces forcommunicating digital signals, and in at least one embodiment, relatesto circuit boards having traces for communicating high-speed digitalsignals and high speed digital differential signals. High-speed digitalsignals may include signals having transition rates up to and exceedingone giga-transition per second, and may even include signals havingtransition rates of greater than ten giga-transitions per second.Although this detailed description specifically refers to severalembodiments of the present invention for communication of digitaldifferential signals, these embodiments are equally suitable to thecommunication of other types of digital signals, including signals whichdo not have out-of-phase components. FIG. 2 illustrates a cross-sectionof a portion of a circuit board having traces for communicatinghigh-speed digital differential signals in accordance with oneembodiment of the present invention. Circuit board 200 includesinsulating substrate 204 having a pair of traces 202 and conductiveground plane 206. Ground plane 206 is located on a side of insulatingsubstrate 204 opposite to trace pair 202.

In the embodiment illustrated in FIG. 2, trace pair 202 includes firstset of conductive traces 208 disposed on a surface of substrate 204, andsecond set of conductive traces 210 which may be parallel to first set208 and disposed within non-conductive material. Second set 210 isseparated from first set 208 by the insulating material of substrate204. In the illustrated embodiment, corresponding traces of first andsecond sets 208, 210 are in a stacked configuration and is referred toas the “stacked trace” embodiment of the present invention. The ends ofeach trace of pair 202 may be electrically coupled together so that thecorresponding traces of the pair function substantially as oneconductor. The corresponding traces of pair 202 may carry a component ofa high-speed digital differential signal.

The dielectric constant (Er) of substrate 204 may be increased over thatof conventional circuit board substrates to compensate for the closerproximity of traces of set 210 to ground plane 206. The dielectricconstant of substrate 204 may, for example, range at least between 3 and4, although substrates and insulating materials having other dielectricconstants may be equally suitable. In the example illustrated, thickness214 of substrate 206 may, for example, range between 20 and 40 microns,width of traces of sets 208 and 210 may range, for example, between 20and 40 microns, and thickness of traces of sets 208 and 210 may range,for example, between 10 and 20 microns. Spacing 212 between traces ofsets 208 and 210 may, for example, be on the order of ten microns orless. The present invention is equally suitable to circuit boards andtraces having different dimensions and characteristics.

FIG. 3 illustrates a cross-section of a portion of a circuit boardhaving traces for communicating high-speed digital differential signalsin accordance with another embodiment of the present invention. Circuitboard 300 includes insulating substrate 304 having trace pair 302 andconductive ground plane 306. Ground plane 306 is located on a side ofinsulating substrate 304 opposite to trace pair 302. Trace pair 302 iscomprised of a trace sets 308 and 310 coupled with conductive material311. The embodiment of the present invention illustrated in FIG. 3 issimilar to circuit board 200 (FIG. 2) except that traces of sets 308 and310 are coupled by conductive material 311. Conductive material 311 mayrun the length of trace pair 302. This embodiment of the presentinvention is referred to as the “I-shaped” embodiment because thecross-section of the traces of pair 302 is similar to a capital “I”.Traces of sets 308 and 310 may be spaced closely together, and may beseparated by a thin layer of insulating material.

FIG. 4 illustrates a cross-section of a portion of a circuit boardhaving traces for communicating high-speed digital differential signalsin accordance with another embodiment of the present invention. Circuitboard 400 includes insulating substrate 404 having trace pair 402 andconductive ground plane 406. Trace pair 402 includes trace set 410located within substrate 404, and conductive portions 411 which maycouple traces of set 410 throughout their length. Conductive portions411 may have a portion exposed on the surface of substrate 404. Thisembodiment of the present invention is referred to as the “T-shaped”embodiment because the cross-section of the traces of pair 402 issimilar to an inverted capital “T”.

FIG. 5 illustrates a cross-section of a portion of a circuit boardhaving traces for communicating high-speed digital differential signalsin accordance with another embodiment of the present invention. Circuitboard 500 includes insulating substrate 504 having trace pair 502 andconductive ground plane 506. Trace pair 502 includes trace set 510located within substrate 504, and conductive portions 511 which maycouple traces of set 510 throughout their length. Conductive portions511 may have a portion exposed on the surface of substrate 504. Thisembodiment of the present invention is referred to as the “L-shaped”embodiment because the cross-section of the traces of pair 502 issimilar to a capital “L”. In this embodiment, conductive portions 511may be positioned on the near sides of traces 510 as illustrated, oralternatively may be positioned on the far sides of traces 510.

FIG. 6 illustrates a cross-section of a portion of a circuit boardhaving traces for communicating high-speed digital differential signalsin accordance with another embodiment of the present invention. Circuitboard 600 includes insulating substrate 604 having trace pair 602 andconductive ground plane 606. Trace pair 602 includes traces of set 610located within substrate 604, traces of set 608 located on a surface ofsubstrate 604, and conductive portions 611 which may couple traces ofsets 610 and 608 throughout their length. Traces of sets 608 and 610 maybe spaced very closely together. This embodiment of the presentinvention is referred to as the “U-shaped” embodiment because thecross-section of the traces of pair 602 is similar to a sideways capital“U”. In this embodiment, conductive portions 611 may be positioned onthe near sides of traces 610 and 608 as illustrated, or alternativelymay be positioned on the far sides of traces 610 and 608 (notillustrated).

The trace pairs of the various embodiments of the present invention mayprovide for reduced conduction loss over conventional trace pairs usedto carry digital differential signals. For example, the conventionaltrace pair for communicating digital differential signals illustrated inFIG. 1 may have a DC resistance of approximately 0.22 Ohms per 2.54centimeters and an AC resistance of approximated 0.33 Ohms per 2.54centimeters. The “stacked trace” embodiment illustrated in FIG. 2 mayhave a DC resistance of approximately 0.11 Ohms per 2.54 centimeters andan AC resistance of approximated 0.21 Ohms per 2.54 centimeters. The“I-shaped” embodiment illustrated in FIG. 3 may have a DC resistance ofapproximately 0.07 Ohms per 2.54 centimeters and an AC resistance ofapproximated 0.20 Ohms per 2.54 centimeters. The “T-shaped” embodimentillustrated in FIG. 4 may have a DC resistance of approximately 0.11Ohms per 2.54 centimeters and an AC resistance of approximated 0.21 Ohmsper 2.54 centimeters. The “L-shaped” embodiment illustrated in FIG. 5may have a DC resistance of approximately 0.14 Ohms per 2.54 centimetersand an AC resistance of approximated 0.24 Ohms per 2.54 centimeters. The“U-shaped” embodiment illustrated in FIG. 6 may have a DC resistance ofapproximately 0.086 Ohms per 2.54 centimeters and an AC resistance ofapproximated 0.19 Ohms per 2.54 centimeters.

The trace pairs of the various embodiments of the present invention mayalso provide for improved signal integrity over conventional trace pairsused to carry digital differential signals. For example, when measuredand viewed with an eye diagram, the percentage at which the signal is ata final state (e.g., not in transition between states) is the percentageof the eye diagram. The percentage of the eye diagram is approximately74.5% for the conventional trace pair of FIG. 1 for a digitaldifferential signal having a transition-rate around ten giga-transitionsper second. The percentage of the eye diagram, for example, isapproximately 86.8% for the “L-shaped” embodiment illustrated in FIG. 5.The percentage of the eye diagram, for example, is approximately 88.8%for the “U-shaped” embodiment illustrated in FIG. 6.

The present invention is not limited to the illustrated embodiments.Other embodiments of the present invention that are not illustratedinclude, for example, a “non-inverted T-shaped” embodiment and an“inverted L-shaped” embodiment.

The trace configurations of the present invention may be suitable forcarrying high-speed digital differential signals in a package to andfrom a semiconductor die. The trace configurations of the presentinvention may also be suitable for carrying high-speed digitaldifferential signals on an interposer substrate, which may be locatedbetween a packaged die and a circuit board. The trace configurations ofthe present invention may also be suitable for carrying high-speeddigital differential signals on circuit boards such as a motherboard ofa computer. The trace configurations of the present invention may alsobe suitable for carrying high-speed digital differential signalsbetween, for example, a high-speed microprocessor and other elements ona motherboard. The trace configurations of the present invention mayalso be suitable for carrying high-speed digital differential signals tovarious I/O elements of a computer system.

FIGS. 7A, 7B and 7C illustrate the fabrication of a circuit board inaccordance with an embodiment of the present invention. FIGS. 7A, 7B and7C may be part of a procedure for fabrication of a circuit board inaccordance with several embodiments of the present invention. Althoughthe individual operations are illustrated and described as separateoperations, it should be noted that one or more of the individualoperations may be performed concurrently. The procedure is described forthe fabrication of a trace pair for carrying a high-speed digitaldifferential signal, however, many more trace pairs for carrying otherhigh-speed digital differential signals may be concurrently fabricated.Although FIGS. 7A, 7B and 7C illustrate the fabrication of the“I-shaped” embodiment described in FIG. 3, this procedure may be used tofabricate other embodiments of the present invention.

In FIG. 7A, trace pair 710 is fabricated on a circuit board havinginsulating substrate 704 and ground plane 706 on a side opposite oftrace pairs 710. In FIG. 7B, insulating material 705 is added tosubstrate 704. Insulating material may be a thin layer of insulatingmaterial, and may be the same or similar to the insulating material ofsubstrate 704. As part of the operation illustrated in FIG. 7B, trenches713 may be formed through insulating material 705. The position oftrenches 713 depends on whether the “I-shaped” embodiment, the“T-shaped” embodiment, the “L-shaped” embodiment, or the “U-shaped”embodiment is being fabricated. When fabricating the “stacked trace”embodiment, trenches 713 do not need to be formed, however “holes” forconductive paths through insulating material 705 may be formed at ornear the ends of the traces of trace pair 710.

In FIG. 7C, trenches 713 are filled with conductive material 711 tocomplete the “T-shaped” and the “L-shaped” embodiment. For the “stackedtrace” embodiment, the holes near the end of traces 710 are filled withconductive material. For the “I-shaped” embodiment, the “sidewaysU-shaped” embodiment, and the “stacked trace” embodiment, traces 708 arefabricated on the surface of insulating material 705.

Various conductive materials may be used for traces 708, traces 710,ground plane 706, and conductive material 711 including, for example,gold, copper, and aluminum, and various alloys and combinations thereof.Various insulating materials may also be used for substrate 704 andinsulating material 705 including, for example, dielectric materialhaving a dielectric constant (Er) ranging at least from 3 to 4, and mayinclude materials such as Duroid, and may also include materials havingdifferent dielectric constants such as aluminia. Although insulatingmaterial 705 may be the same or similar to the material of substrate704, this is not a requirement: insulating material 705 may be differentfrom the material of substrate 704.

FIG. 8 is a highly simplified functional block diagram of a system inaccordance with an embodiment of the present invention. System 800comprises first and second communication elements 802, 806, withcommunication path 804 therebetween. First communication element 802 mayprovide digital differential signal 808, and second communicationelement 806 may receive digital differential signal 808. Communicationpath 804 may carry the digital differential signal between first andsecond communication elements 802, 806. Communication path 804 mayinclude a first set of conductive traces disposed on insulatingmaterial, and a second set of conductive traces parallel to the firstset and disposed within the insulating material and may be separatedfrom the first set by the insulating material. In one embodiment,corresponding traces of the first and second sets may be in a stackedconfiguration. The corresponding traces of the first and second sets maycarry a portion of the digital differential signal, and may beelectrically coupled at one end. The first and second communicationelements may part of a communication system, a computer system, or othersystem that communicates digital differential signals between elements.

The trace configurations illustrated in FIGS. 2-7 are suitable for useas part of communication path 804, although other trace configurationsare also suitable. Corresponding traces of the first and second sets mayhave spacing therebetween of less than twenty microns. The communicationpath may include conductive material to electrically couple thecorresponding traces of the first and second sets. The conductivematerial may be disposed in between the corresponding traces of thefirst and second sets for substantially an entire length of thecorresponding traces.

Thus, trace configurations for improved communication of high-speeddigital differential signals have been described. Circuit boards havingtraces suitable for communication of high-speed digital differentialsignals have also been described. The traces may communicate high-speeddigital differential signals with reduced conduction loss. The tracesmay also communicate high-speed digital differential signals withreduced dielectric loss. The traces may also communicate high-speeddigital differential signals while maintaining signal integrity.

The foregoing description of the specific embodiments reveals thegeneral nature of the invention sufficiently that others can, byapplying current knowledge, readily modify and/or adapt it for variousapplications without departing from the generic concept, and thereforesuch adaptations and modifications are intended to be comprehendedwithin the meaning and range of equivalents of the disclosedembodiments. It is to be understood that the phraseology or terminologyemployed herein is for the purpose of description and not of limitation.Accordingly, the invention is intended to embrace all such alternatives,modifications, equivalents and variations as fall within the spirit andbroad scope of the appended claims.

1. A circuit board comprising: a first set of conductive traces disposedon insulating material; a second set of conductive traces parallel tothe first set and disposed within the insulating material and separatedfrom the first set by the insulating material; and conductive materialto electrically couple the corresponding traces of the first and secondsets, the conductive material being disposed in between thecorresponding traces of the first and second sets for substantially anentire length of the corresponding traces, wherein the correspondingtraces of the first and second sets including the conductive materialdisposed therebetween have a cross-section similar to an “I” shape, andwherein the corresponding traces of the first and second sets carry aportion of a digital signal.
 2. The circuit board of claim 1 wherein theinsulating material has a dielectric constant (Er) ranging approximatelybetween 3 and
 4. 3. The circuit board of claim 1 wherein the digitalsignal is a high-speed digital differential signal having atransition-rate of at least one giga-transition per second.
 4. Thecircuit board of claim 1 wherein the conductive traces of the first andsecond sets are either copper or a copper alloy.
 5. The circuit board ofclaim 1 further comprising a ground plane, and wherein the first andsecond sets of conductive traces run substantially parallel to theground plane.
 6. The circuit board of claim 5 wherein the ground planeis copper and is on a side of the non-conductive material opposite fromthe second set of conductive traces.
 7. A circuit board forcommunicating a high-speed digital signal comprising: insulatingmaterial; a first set of conductive traces disposed on the insulatingmaterial; a second set of conductive traces parallel to the first setand disposed within the insulating material and separated from the firstset by the insulating material; and conductive material to electricallycouple the corresponding traces of the first and second sets, theconductive material being disposed in between the corresponding tracesof the first and second sets for substantially an entire length of thecorresponding traces, wherein the corresponding traces of the first andsecond sets including the conductive material disposed therebetween havea cross-section similar to an “I” shape, and wherein the correspondingtraces of the first and second sets carry a portion of a digital signal.8. The circuit board of claim 7 wherein the insulating material has adielectric constant (Er) ranging approximately between 3 and
 4. 9. Thecircuit board of claim 7 wherein the digital signal is a high-speeddigital differential signal having a transition-rate of at least onegiga-transition per second.
 10. The circuit board of claim 7 wherein theconductive traces are comprised of either copper or a copper alloy. 11.An interposer substrate comprising: insulating material; a first set ofconductive traces disposed on insulating material; a second set ofconductive traces parallel to the first set and disposed within theinsulating material and separated from the first set by the insulatingmaterial; and conductive material to electrically couple thecorresponding traces of the first and second sets, the conductivematerial being disposed in between the corresponding traces of the firstand second sets for substantially an entire length of the correspondingtraces, wherein the corresponding traces of the first and second setsincluding the conductive material disposed therebetween have across-section similar to an “I” shape, and wherein the correspondingtraces of the first and second sets carry a portion of a digital signal.12. The interposer substrate of claim 11 further comprising a groundplane, and wherein the pair of conductive traces run substantiallyparallel to the ground plane.
 13. The interposer substrate of claim 11wherein the digital signal is a high-speed digital differential signalhaving a transition-rate of at least one giga- transition per second.14. A package for a semiconductor die: insulating material; and a firstset of conductive traces disposed on insulating material; a second setof conductive traces parallel to the first set and disposed within theinsulating material and separated from the first set by the insulatingmaterial; and conductive material to electrically couple thecorresponding traces of the first and second sets, the conductivematerial being disposed in between the corresponding traces of the firstand second sets for substantially an entire length of the correspondingtraces, wherein the corresponding traces of the first and second setsincluding the conductive material disposed therebetween have across-section similar to an “I” shape, and wherein the correspondingtraces of the first and second sets carry a portion of a digital signal.15. The package of claim 14 further comprising a ground plane, andwherein the pair of conductive traces run substantially parallel to theground plane.
 16. The package of claim 14 wherein the digital signal isa digital differential signal having a transition-rate of at least onegiga-transition per second.
 17. A system comprising: a firstcommunication element to provide a digital signal; a secondcommunication element to receive the digital signal; and a communicationpath between the first and second communication elements to carry thedigital differential signal, the communication path comprising: a firstset of conductive traces disposed on insulating material; a second setof conductive traces parallel to the first set and disposed within theinsulating material and separated from the first set by the insulatingmaterial; and conductive material to electrically couple thecorresponding traces of the first and second sets, the conductivematerial being disposed in between the corresponding traces of the firstand second sets for substantially an entire length of the correspondingtraces, wherein the corresponding traces of the first and second setsincluding the conductive material disposed therebetween have across-section similar to an “I” shape, and wherein the correspondingtraces of the first and second sets carry a portion of a digital signal.18. The system of claim 17 wherein the digital signal is a high-speeddigital differential signal having a transition rate of at least onegiga-transition per second and the first and second communicationelements are part of a computer system.
 19. The system of claim 18wherein corresponding traces of the first and second sets have a spacingtherebetween of less than twenty microns.